1. Field of the Invention
The present invention relates to a method of producing a micro-electromechanical element, and, more specifically, to a micro-electromechanical element which is implemented such that a micromechanical structure and electronic components are arranged in the same semiconductor wafer.
2. Description of Prior Art
Due to the fast development in the field of semiconductor industry and microelectronics, micromechanical elements, e.g. silicon-based micromechanical pressure measurement cells, replace classical mechanical pressure transducers more and more. Micromechanical elements are used in great amounts e.g. in the fields of automation technology and medical engineering as well as in automotive vehicles. The systems preferably used in this connection are micro-electromechanical integrated systems which realize the combination of mechanical and electronic functions on one substrate. In addition to the electronic components produced in CMOS or similar technologies, which are e.g. measuring transducers, amplifiers, storage means, microcontrollers, etc., mechanical components exist in the same layers. These mechanical components may e.g. be diaphragms of pressure sensors, elastic sheets of valves or pumps, oscillating masses of acceleration sensors, movable fingers or cantilevered arms of switches and the like. In addition to the more or less smooth surface which normally exists in the field of planar technology, these mechanical structures have a three-dimensional structural design and comprise cantilevered structures and buried cavities.
Up to now, it has been known to produce such cavities by wet-chemical etching or by connecting two disks which have been fully processed individually in advance. The techniques which may be used as a connection technique for connecting the individually processed disks are wafer bonding (silicon fusion bonding), anodic bonding or glueing. In any case, the individual disks are fully processed before they are connected in accordance with conventional methods, so that processing steps which may impair the mechanical structures need not be carried out afterwards.
When a wet-chemical undercutting of structures is carried out, the so-called “sticking” problem arises in the case of which the cantilevered structure will adhere to the neighbouring surface due to capillary forces occurring when the liquid dries; hence, the cantilevered structure will lose its movability. Small ditches, holes and gaps additionally cause problems when the structure in question is to be wetted with liquids (e.g. etching solutions, cleaning water, photoresist) and during the removal of these liquids, the problems being then caused e.g. by bubbles which may adhere to the structure in corners thereof. In the case of spinning, drops may remain, which will cause marks when they dry up. Cleaning by means of brushes is problematic as well, since the movable structures may break off during such cleaning processes. A clean surface of the structures is, however, necessary so that the production methods for producing the evaluation structures, such as a CMOS method, can be applied. Due to the risk of carrying over particles and contaminations, the CMOS ability may no longer be given in the case of open structures, i.e. certain sequences of process steps are not allowed in an CMOS line. In addition, when the chips comprised in the wafer are diced by means of a wafer saw, water is used as a rinsing liquid, which may penetrate into open cavities thus aggravating the particle and contamination problem.
It is additionally known to produce diaphragm-like structures making use of KOH back etching, when the electronic components on the front surface of a wafer have been finished. Due to the oblique etch edges occurring in the case of KOH etching, the integration level will, however, decrease substantially when this method is used, especially when a high number of micro-electromechanical components is produced from one wafer.
Various methods of producing semiconductor pressure sensors are additionally described in DE 3743080 A1.
DE-C-19543893 describes a method of aligning structures which are to be produced in a substrate, in the case of which a diaphragm-like structure is formed on top of a cavity. For this purpose, a closed cavity is first produced between two substrates by forming a recess in a masking layer on one of the substrates, whereupon the two substrates are interconnected via the masking layer and one of the substrates is thinned in a final step.
U.S. Pat. No. 4,586,109 describes a method of producing a capacitive pressure sensor. For this purpose, a layer arranged on a substrate is first structured so that a closed cavity will be formed, when the substrate has been connected via this layer to a further substrate. This cavity can then be opened through one of the substrates for producing a relative pressure sensor.
EP-A-639761 discloses a method of producing a differential pressure sensor in the case of which a substrate structure, in which a micromechanical element is formed, is connected to a glass substrate.